How I put my DS on my back in a ring sling is to start with the rings on my bag and the tail on my shoulder (so I can reach it) I then put him in like a normal hip carry, make sure there is a nice deep pocket and that the bottom rail is right up under the knee. Then as I tighten I also bounce DS towards the back of my hip (under my arm) Make sure the top rail is nice and snug. Adjust shoulder and tighten some more. It is a hip carry but instead of on the front of your hip he sits on the back of your hip.
I hope that makes sense. I can only wear DS for a short time this way, like doing dishes, carrying groceries, cooking etc. But it works great when you need your to hands. Try doing a Youtube search of ring sling back carry. Good luck!